The invention relates to an electronic control circuit for adjusting the control voltage of a device to be controlled, the control circuit comprising a primary coil, a control bus comprising a first secondary coil, a first control diode, a first capacitor and means for adjusting the control voltage, the means being parallel-connected with the first capacitor, the parallel connection being further series-connected with the first secondary coil and the first control diode, and a control voltage supply circuit comprising a series-connected second secondary coil, a second control diode and a second capacitor.
In the present specification, the term diode refers to any electronic component conducting current in one direction only and providing a diode-like effect. It is obvious to a person skilled in the art that this can be implemented by a transistor, for example. In the same way, in the present specification, the term capacitor refers to any capacitive element which is electrically chargeable in the same way as a capacitor.
Electronic control loops and circuits commonly employ a separate control unit which often requires galvanic separation from the equipment to be controlled. Galvanic separation enables a sufficient electric separation between different electronic circuits and yet at the same time transmits a voltage signal from one electronic circuit to another. Galvanic separation is implemented by either optical or magnetic components.
The use of a 1 to 10 volt direct-current voltage as the control voltage has become more common in many electronic control circuits, particularly in lighting control systems. In this case a 10 V control voltage creates a maximum light level and a 1 V control voltage a minimum light level. Minimum and maximum light levels can preferably be freely selected and adjusting the control voltage allows the light level to be changed steplessly between minimum and maximum values. Usually the operating voltage of a control unit is directly supplied from the power source of the device to be controlled, the power source supplying current to the control unit via a control bus. This solution enables a simple implementation for a control unit, whereby the control unit does not necessarily require external operating voltage. Such a control principle is commonly used for example in adjusting electronic connectors in fluorescent lamps, phase angle controllers and electronic halogen and neon lamp transformers.
A control circuit is often implemented by the connection shown in FIG. 1. The connection comprises a control transformer T1 having three coils N1, N2 and N3. N1 is the primary coil of the transformer, N2 the secondary coil of a control bus 1 and N3 the secondary coil of a device to be controlled. The control bus 1 further comprises a diode D1, a adjustable zener diode Z1 and a capacitor C1. The diode D1 is series-connected with the secondary coil N2 of the control bus 1. The zener diode Z1 and the capacitor C1 are parallel-connected, the paralleling, in turn, being series-connected with the secondary coil N2 of the control bus 1 and the diode D1. In a control voltage supply circuit 2, the secondary coil N3 of the device to be controlled is series-connected with the diode D2 and the capacitor C2. A switch K1 is coupled to the primary coil N1 of the transformer, and opened and closed under the control of a control block A. The operation of the control block A is known per se to a person skilled in the art, and does not need to be discussed in any greater detail herein.
The connection of the control circuit is what is known as a forced flyback connection. As the control block A closes the switch K1, a magnetization current starts to flow in the primary coil N1 of the transformer T1. The magnitude of the magnetization current varies substantially between 5 and 100 mA. The operating current of the control block A is typically between 3 and 5 mA. The coiling directions of the coils in the transformer T1 are so selected that the ends of the secondary coils N2 and N3 on the side of the diodes D1 and D2 are negative when the magnetization current is flowing, whereby no current flows in the secondary coils N2 and N3. The level of the control voltage is controlled by an adjustable zener diode Z1. When the control block A opens the switch K1, the magnetization energy stored in the ferrite of the transformer T1 causes a current in the secondary coils N2 and N3 charging the capacitors C1 and C2. The magnitude of the voltage Uc over the capacitor C1 is adjusted by the zener diode Z1. In this case, provided the secondary coils N2 and N3 have an identical number of turns, the control voltage Ue of the device to be controlled is equal to the voltage Uc, i.e. Ue=Uc. This way the voltage level, adjusted by the zener diode Z1, for controlling the light level, has been transmitted magnetically.
In accordance with prior art, the control circuit connection can be also implemented by a connection according to FIG. 2. The connection in FIG. 2 is what is known as a blocking oscillator, in which the control block A and the switch K1 have been replaced by a transistor V1, resistors R1, R2 and R3 and a capacitor C3 as compared with the connection in FIG. 1. Together with a coil N1, these form an oscillation circuit in such a way that the coil N1 is connected to the emitter of the transistor V1, the resistors R1 and R2, the coil N3 and the resistor R3 are parallel-connected with these to the operating voltage, and the capacitor C3 is parallel-connected with the resistors R1 and R2 and the coil N3. The filtering capacitor C2 is prevented from being charged by connecting it with a reverse-biased diode D2 between the transistor V1 and the coil N1. The base current of the transistor can be taken preferably from between the resistors R1 and R2, for example.
The base current of the transistor V1 flows via the resistor R2, the coil N3 and the resistor R3 and brings the transistor V1 to a saturation state, whereby the operation of the transistor V1 corresponds to a closed switch, and as a result the coil N1 is coupled via the transistor V1 to the operating voltage Vcc. The current passing through the coil N1 makes the coil N1 operate as a primary coil with respect to N3, whereby an increasing voltage in N3 controls more strongly the transistor V1 to a saturation state. When the current passing through the coil N1 increases so high that the base current is no longer sufficient to keep the transistor V1 in a saturation state, the direction of the current passing through the transistor V1 turns in an opposite direction. As the voltage over the coil N1 decreases, the base current also decreases, making the transistor V1 an opened switch. An opposite current direction opens the diode D2, whereby a negative control voltage Ue charges over the capacitor C2 and has a magnitude which is determined by the relation between the number of turns of the coils N1 and N2, i.e. Ue=(xe2x88x92N1/N2)*Uc.
In other words, in prior art solutions, the magnetization current of the primary coil is taken from the operating voltage of the control electronics of the device to be controlled, the voltage being typically between 10 and 15 V. In this case, if the control current is 1 mA, a typical value for the control current, the output level is correspondingly (10-15 V)*1 mA=10 to 15 mW. The efficiency of the connection in FIG. 1 is about 0.5 and that of the connection in FIG. 2 about 0.2. In this case the power consumption of the connections is 2 mA and 5 mA, respectively. in addition, in the connection according to FIG. 1, the control block A typically consumes between 3 and 5 mA of current.
However, prior art solutions show clear drawbacks. In both of the above connections the power source of the device to be controlled also operates as the power source of the control circuit, which further increases power consumption. In the connection of FIG. 1, the control block A needs an individual operating current. In both connections, the transformer T1 needs significantly much space as compared with the space required by the entire control circuit. The size of the transformer is influenced mainly by isolation class and the space taken up by the coils. Also, when a plurality of turns are required, the amount of coiling work naturally also increases. From the point of view of the operation, the use of a small toroidal or E core body is advantageous at a frequency of about 20 kHz, for example, and the required number of turns in the coils are in the order of 15/10/10 (N1/N2/N3) in the connection of FIG. 1 and 10/10/3 in the connection of FIG. 2, respectively.
It is the object of the present invention to provide a control circuit avoiding the above drawbacks. To be more exact, the control circuit of the invention is characterized by the primary coil being connected between a first node and a second node of the device to be controlled, and the nodes being selected such that the current in an electric circuit between them at least momentarily reaches the value zero. It is an essential idea of the invention to achieve primary coil magnetization current without separate control electronics, but to have a power supply in the device to be controlled generate the magnetization current. It is the idea of another preferred embodiment of the invention that one primary coil turn is sufficient because of the high value of the magnetization current.
It is an advantage of the invention that in the control circuit of the invention, the number of components is lower, resulting in a simpler connection. The primary coil has one turn of wiring only, and this is a further advantage of the invention, requiring less coiling work and enabling a substantially smaller transformer size. Furthermore, the solution of the invention brings about power savings, since the magnetization current is taken directly from the device to be controlled.